If the Imam has not had the intention of leading the prayer and then some persons join him and he leads them

Sahih al-Bukhari · Book 10 · Hadith 699

حَدَّثَنَا مُسَدَّدٌ، قَالَ حَدَّثَنَا إِسْمَاعِيلُ بْنُ إِبْرَاهِيمَ، عَنْ أَيُّوبَ، عَنْ عَبْدِ اللَّهِ بْنِ سَعِيدِ بْنِ جُبَيْرٍ، عَنْ أَبِيهِ، عَنِ ابْنِ عَبَّاسٍ، قَالَ بِتُّ عِنْدَ خَالَتِي فَقَامَ النَّبِيُّ صلى الله عليه وسلم يُصَلِّي مِنَ اللَّيْلِ، فَقُمْتُ أُصَلِّي مَعَهُ فَقُمْتُ عَنْ يَسَارِهِ، فَأَخَذَ بِرَأْسِي فَأَقَامَنِي عَنْ يَمِينِهِ‏.‏

Narrated Ibn `Abbas: Once I passed the night in the house of my aunt Maimuna. The Prophet (ﷺ) stood for the night prayer and I joined him and stood on his left side but he drew me to his right by holding me by the head.